Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1999-09-07
2002-08-27
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S077040
Reexamination Certificate
active
06441986
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to disk drive data storage devices. More particularly, the present invention relates to methods for improving the tracking capabilities of a servo system for positioning a read/write head with respect to a track on a disk drive data storage device.
BACKGROUND OF THE INVENTION
In conventional compute data storage systems having a rotating storage medium, data is stored in a series of concentric or spiral tracks across the surface of a disk. Each track includes a number of data sectors. The storage medium can comprise, for example, a disk having a surface on which a magnetic material is deposited, such as conventional magnetic disks or magneto-optical disks. The data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits are read from the disk surface by a head transducer suspended over the disk surface that can detect the variations in magnetic orientation as the disk rotates relative to the head and generate a fluctuating data signal.
Conventionally, a magnetic head is mounted on an arm or carriage that is incorporated in a servo system that moves the head, via an actuator, in a “seek” or “access” function; i.e., the servo system moves the head to a selected track from a previous track. When the head reaches the desired track, the servo system commences a “track following” function in which it accurately positions the head over the centerline of the selected track and maintains it in that position so that the head can write a series of data bits or, alternatively, read a series of bits from the track as the disk rotates under the head. Thus, the disk drive servo control system controls movement of the arm across the surface of the disk to move the head from track to track and, once over a selected track, to maintain the magnetic head within a given tolerance of distance over the centerline of the desired data track during read or write operations.
One such system is a sectored servo control system that is used to maintain the magnetic read/write head precisely over a desired track during a read or write operation. Such a servo system requires that servo information be prerecorded on the disk file. Servo information can be prerecorded on servo sectors located on each disk surface.
During both seeking and track following operations, the prerecorded servo information is sensed by the head and demodulated to generate a digital gray code and a position error signal (PES). The digital gray code includes track identification information and the PES indicates the position of the head away from the centerline of the track (i.e., an offset from the center of the track). The digital gray code and the PES are combined together to generate a measured position signal. The measured position signal is then used in a servo feedback loop to generate a control signal to move the head along a desired seek trajectory or towards the centerline of the target track.
In other words, each disk stores servo information in different sectors of the disk required for positional control. The embedded sectored servo method reproduces servo information written on the disk to determine the track number and the exact position of the head relative to the center of the track. A description of a general digital disk file servo control system is given by U.S. Pat. No. 4,679,103 granted to Michael I. Workman and titled “Digital Servo Control System For a Data Recording Disk File”.
Embedded servo bursts are angularly spaced pie-piece-shaped sectors that are interspersed among the data sectors on the data disks. Alternatively, the servo signals may be recorded on a disk surface dedicated to servo signals. All of these mechanisms for servo control information are well known to those of ordinary skill in the art. The prerecorded servo information is normally written as servo sectors or bursts to the disks by a device commonly referred to as a “servowriter” or “trackwriter” in a factory environment, before the disks are shipped to users. The servowriter is a machine dedicated to writing servo signals onto the disk's surface. After the servowriter has recorded the servo information in the servo sectors, the disk is checked for quality by verifying the accuracy of the servo information. All of these mechanisms for servo control information are well known to those of ordinary skill in the art.
When a disk is used in a disk drive, the prerecorded servo information, including servo bursts, is read from the disks, demodulated, and processed by the servo control system. The results are then applied to the input of the servo electronics which in turn controls the current to an actuator such as a voice coil motor (VCM) and thus controls the position of the head over the disk surface.
Conventionally, A/B servo bursts are recorded at regularly spaced angular intervals around the disk surface. The servo burst fields are written symmetrically offset from and on respective sides of the data track center line by at least one-half of the track width. That is, the servo bursts do not overlap the track center line. Consequently, the difference between the relative voltage amplitudes, (V
A
-V
B
), as read by the head while track following, may be utilized as a direct indication of the distance and direction of the head from the track center line. In other words, when the drive head passes over these bursts, the readback amplitude of these bursts will be proportional to the portion of the width of the head that physically passes over the burst. For a head exactly on-track, both A and B bursts will read back with identical amplitude. If the head is biased toward the outside of the track, burst B will have higher amplitude than burst A, for example. If the head is biased toward the inside of the track, burst A will have higher amplitude, for example. By detecting the amplitude of A and B individually, and by calculating the difference between the amplitudes, the off-track PES signal can be generated.
Some disk drives accept removable disk cartridges that contain a magnetic storage medium upon which information can be written and read. The disk-shaped storage medium is mounted on a hub and both rotate freely as a unit within the cartridge. In order to rotate the disk, a spindle motor within the drive engages the disk hub when the cartridge is inserted into the drive. The spindle motor does this by first moving from an unloaded position to a loaded position when a disk cartridge is inserted into the disk drive. In the loaded position, the spindle of the disk drive motor contacts the hub of the disk cartridge. The spindle can then be rotated in order to rotate the hub and the storage medium of the disk cartridge. A shutter on the front edge of the cartridge is moved to the side during insertion into the drive, thereby exposing an opening through which the read/write heads of the drive move to access the recording surfaces of the rotating storage medium.
There are several causes for the position of a read/write head to be in error, or off-track, during a track following operation. One of the major components of head position error is called repeatable runout (RRO). RRO that occurs at the disk rotating frequency may be called 1F runout. There are several possible causes for
1
F runout, such as an unbalanced spindle, a non-ideal spindle bearing, or centering error of a removable disk. The Workman U.S. Pat. No. 4,679,103 does not specifically deal with this problem. The servo control system disclosed in the Workman U.S. Pat. No. 4,679,103 does not have sufficient gain at the runout frequency to correct for the RRO error.
Particularly where a disk is removable from the drive, there is the possibility that the center of the circular tracks is not coincident with the center of the drive motor spindle, due to mechanical clearances causing a centering
Bero James M.
Ma Yiping
Moon Lawrence M.
Wright Kelly D.
Hudspeth David
Iomega Corporation
Wong K.
Woodcock & Washburn LLP
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